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A 6 year old is prepared to go under anaesthesia
A 6 year old is prepared to go under anaesthesia
Anesthesia, or anaesthesia (from Greek ἀν-, an-, "without"; and αἴσθησις, aisthēsis, "sensation", see spelling differences) is a temporary state consisting of unconsciousness, loss of memory, lack of pain, and muscle relaxation.
Anesthesia is a unique medical intervention which does not itself offer any particular medical benefit and instead enables the performance of other medical interventions. The best anesthetic is therefore one with the lowest risk to the patient that still achieves the end points required to complete the other intervention. There are many different needs and goals of anesthesia. The goals (end points) are traditionally described as unconsciousness and amnesia, analgesia, and muscle relaxation. To reach multiple end points one or more drugs are commonly used (such as general anesthetics, hypnotics, sedatives, paralytics, narcotics, and analgesics), each of which serves a specific purpose in creating a safe anesthetic.
The types of anesthesia are broadly classified into general anesthesia, sedation and regional anesthesia. General anesthesia refers to the suppression of activity in the central nervous system, resulting in unconsciousness and total lack of sensation. Sedation (or dissociative anesthesia) uses agents that inhibit transmission of nerve impulses between higher and lower centers of the brain inhibiting anxiety and the creation of long-term memories. Regional anesthesia renders a larger area of the body insensate by blocking transmission of nerve impulses between a part of the body and the spinal cord. It is divided into peripheral and central blockades. Peripheral blockade inhibits sensory perception within a specific location on the body, such as when a tooth is "numbed" or when a nerve block is given to stop sensation from an entire limb. Central blockades place the local anesthetic around the spinal cord (such as with spinal and epidural anesthesia) removing sensation to any area below the level of the block.
There are both major and minor risks of anesthesia. Examples of major risks include death, heart attack and pulmonary embolism whereas minor risks can include postoperative nausea and vomiting and readmission to hospital. The likelihood of a complication occurring is proportional to the relative risk of a variety of factors related to the patient's health, the complexity of the surgery being performed and the type of anesthetic. Of these factors, the person's health prior to surgery (stratified by the ASA physical status classification system) has the greatest bearing on the probability of a complication occurring. Patients typically wake within minutes of an anesthetic being terminated and regain their senses within hours. One exception is a condition called long-term post-operative cognitive dysfunction, characterized by persistent confusion lasting weeks or months, which is more common in those undergoing cardiac surgery and in the elderly.
The first documented general anesthetic was performed by Crawford W. Long in 1842. Unfortunately for Long, he did not publish his successes with ether for general anesthesia until 1849. The first public demonstration of general anesthesia was in 1846 by a Boston dentist named William T.G. Morton at the Massachusetts General Hospital. Dr. Morton gave an ether anesthetic for the removal of a neck tumor by surgeon John Collins Warren (the first editor of the New England Journal of Medicine and dean of Harvard Medical School). About a decade later, cocaine was introduced as the first viable local anesthetic. It wasn't until the 1930s that Dr. Harvey Cushing tied the stress response to higher mortality rates and began using local anesthetic for hernia repairs in addition to general anesthesia.
The purpose of anesthesia can be distilled down to three basic goals or end points::236
Different types of anesthesia (which are discussed in the following sections) affect the endpoints in different ways. Regional anesthesia, for instance affects analgesia, benzodiazepine type sedatives (used in twilight sleep) favor amnesia and general anesthetics can affect all of the endpoints. The goal of anesthesia is to achieve the necessary endpoints with the least amount of risk possible to the patient.
To achieve the goals of anesthesia, drugs act on different but interconnected parts of the nervous system. Hypnosis, for instance, is generating through actions on the nuclei in the brain and is similar to the activation of sleep. The effect is to make people less aware and less reactive to non-noxious stimuli.:245
Loss of memory (amnesia) is created by action of drugs on multiple (but specific) regions of the brain. Memories are created as either declarative or non-declarative memories in several stages (short-term, long-term, long-lasting) the strength of which is determined by the strength of connections between neurons termed synaptic plasticity.:246 Each anesthetic produces amnesia through unique effects on memory formation at variable doses. Inhalational anesthetics will reliably produce amnesia through general suppression of the nuclei at doses below those required for loss of consciousness. Drugs like midazolam produce amnesia through different pathways by blocking the formation of long-term memories.:249
Tied closely to the concepts of amnesia and hypnosis is the concept of consciousness. Consciousness is the higher order process that synthesizes information. For instance, the “sun” conjures up feelings, memories and a sensation of warmth rather than a description of a round, orange warm ball seen in the sky for part of a 24 hour cycle. Likewise, a person can have dreams (a state of subjective consciousness) during anesthetic or have consciousness of the procedure despite having no indication of it under anesthetic. It is estimated that 22% of people dream during general anesthesia and 1 or 2 cases per 1000 have some consciousness termed “awareness during general anesthesia”.:253
Anesthesia is unique, in that it does not offer any particular benefit, rather it allows others to do things that might be beneficial. The best anesthetic, therefore is the one with the lowest risk to the patient that still achieves the endpoints required to complete the procedure. The first stage of an anesthetic is the pre-operative risk assessment made up of the medical history, physical examination and lab tests. Diagnosing a person's pre-operative physical status allows the clinician to minimize anesthetic risks. A well completed medical history will arrive at the correct diagnosis 56% of the time which increases to 73% with a physical examination. Lab tests help in diagnosis but only in 3% of cases, underscoring the need for a full history and physical examination prior to anesthetics. Incorrect pre-operative assessments or preparations are the root cause of 11% of all adverse anesthetic events.:1003
|ASA class||Physical status|
|ASA 1||Healthy person|
|ASA 2||Mild systemic disease|
|ASA 3||Severe systemic disease|
|ASA 4||Severe systemic disease that is a constant threat to life|
|ASA 5||A moribund person who is not expected to survive without the operation|
|ASA 6||A declared brain-dead person whose organs are being removed for donor purposes|
|E||Suffix added for patients undergoing emergency procedure|
One part of the risk assessment is based on the patients' health. The American Society of Anesthesiologists have developed a six-tier scale which stratifies the pre-operative physical state of the patient called the ASA physical status. The scale assesses a high-order of risk as the patient's general health relates to an anesthetic.
The more detailed pre-operative medical history aims to discover genetic disorders (such as malignant hyperthermia or pseudocholinesterase deficiency), habits (tobacco, drug and alcohol use), physical attributes (such as obesity or a difficult airway) and any coexisting diseases (especially cardiac and respiratory diseases) that might impact the anesthetic. The physical examination helps quantify the impact of anything found in the medical history in addition to lab tests.:1003–1009
Aside from the generalities of the patients health assessment, an evaluation of the specific factors as they relate to the surgery also need to be considered for anesthesia. For instance, anesthesia during childbirth must consider not only the mother but the baby. Cancers and tumors that occupy the lungs or throat create special challenges to general anesthesia. After determining the health of the person undergoing anesthetic and the endpoints that are required to complete the procedure, the type of anesthetic can be selected. Choice of surgical method and anaesthetic technique aims to reduce risk of complications, shorten time needed for recovery and minimise the surgical stress response.
Anesthesia is the combination of the endpoints (discussed above) which are reached by drugs acting on different but overlapping sites in the central nervous system. General anesthesia (as opposed to sedation or regional anesthesia) has three main goals: lack of movement (paralysis), unconsciousness, and blunting of the stress response. In the early days of anesthesia, anesthetics could reliably achieve the first two, allowing surgeons to perform necessary procedures, but many patients died because the extremes of blood pressure and pulse caused by the surgical insult were ultimately harmful. Eventually, the need for blunting of the surgical stress response was identified by Harvey Cushing, who injected local anesthetic prior to hernia repairs.:30 This led to the development of other drugs that could blunt the response leading to lower surgical mortality rates.
The most common approach to reach the endpoints of general anesthesia is through the use of inhaled general anesthetics. Each has its own potency which is correlated to its solubility in oil. This relationship exists because the drugs bind directly to cavities in proteins of the central nervous system, although several theories of general anaesthetic action have been described. Inhalational anesthetics are thought to exact their effects on different parts of the central nervous system. For instance, the immobilizing effect of inhaled anesthetics results from an effect on the spinal cord whereas sedation, hypnosis and amnesia involve sites in the brain.:515 The potency of an inhalational anesthetic is quantified by its minimum alveolar concentration or MAC. The MAC is the percentage dose of anaesthetic that will prevent a response to painful stimulus in 50% of subjects. The higher the MAC, generally, the less potent the anesthetic.
The ideal anesthetic drug would provide hypnosis, amnesia, analgesia, and muscle relaxation without undesirable changes in blood pressure, pulse or breathing. In the 1930s, physicians started to augment inhaled general anesthetics with intravenous general anesthetics. The drugs used in combination offered a better risk profile to the person under anesthetic and a quicker recovery. A combination of drugs was later shown to result in lower odds of dying in the first 7 days after anesthetic. For instance, propofol (injection) might be used to start the anesthetic, fentanyl (injection) used to blunt the stress response, midazolam (injection) given to ensure amnesia and sevoflurane (inhaled) during the procedure to maintain the effects. More recently, several intravenous drugs have been developed which, if desired, allow inhaled general anesthetics to be avoided completely.:720
The core instrument in an inhalational anesthetic delivery system is an anesthetic machine. It has vaporizers, ventilators, an anesthetic breathing circuit, waste gas scavenging system and pressure gauges. The purpose of the anesthetic machine is to provide anesthetic gas at a constant pressure, oxygen for breathing and to remove carbon dioxide or other waste anesthetic gases. Since inhalational aenesthetics are inflammable, various checklists have been developed to confirm that the machine is ready for use, that the safety features are active and the electrical hazards are removed. Intravenous anesthetic is delivered either by bolus doses or an infusion pump. There are also many smaller instruments used in airway management and monitoring the patient. The common thread to modern machinery in this field is the use of fail-safe systems that decrease the odds of catastrophic misuse of the machine.
Patients under general anesthesia must undergo continuous physiological monitoring to ensure safety. In the US, the American Society of Anesthesiologists (ASA) have established minimum monitoring guidelines for patients receiving general anesthesia, regional anesthesia, or sedation. This includes electrocardiography (ECG), heart rate, blood pressure, inspired and expired gases, oxygen saturation of the blood (pulse oximetry), and temperature. In the UK the Association of Anaesthetists (AAGBI) have set minimum monitoring guidelines for general and regional anesthesia. For minor surgery, this generally includes monitoring of heart rate, oxygen saturation, blood pressure, and inspired and expired concentrations for oxygen, carbon dioxide, and inhalational anesthetic agents. For more invasive surgery, monitoring may also include temperature, urine output, blood pressure, central venous pressure, pulmonary artery pressure and pulmonary artery occlusion pressure, cardiac output, cerebral activity, and neuromuscular function. In addition, the operating room environment must be monitored for ambient temperature and humidity, as well as for accumulation of exhaled inhalational anesthetic agents, which might be deleterious to the health of operating room personnel.
Sedation (also referred to as dissociative anesthesia or twilight anesthesia) creates hypnotic, sedative, anxiolytic, amnesic, anticonvulsant, and centrally produced muscle-relaxing properties. From the perspective of the person giving the sedation, the patient will appear sleepy, relaxed and forgetful, allowing unpleasant procedures to be more easily completed. Sedatives such as benzodiazepines are usually given with pain relievers (such as narcotics, or local anesthetics or both) because they don't, by themselves, provide significant pain relief.
From the perspective of the person receiving sedative, the effect is a feeling of general relaxation, forgetfulness and time passing quickly. Many drugs can produce a sedative effect including benzodiazepines, propofol, thiopental, ketamine and inhaled general anesthetics. The advantage of sedation over a general anesethetic is that it generally doesn't require support of the airway or breathing (no tracheal intubation or mechanical ventilation) and can have less of an effect on the cardiovascular system which may add to a greater margin of safety in some patients.:736
When pain is blocked from a part of the body using local anesthetics, it is generally referred to as regional anesthesia. There are many types of regional anesthesia either by injecting into the tissue itself, a vein that feeds the area or around a nerve trunk that supplies sensation to the area. The later are called nerve blocks and are divided into peripheral or central nerve blocks.
The following are the types of regional anesthesia::926–931
When local anesthetic is injected around a larger diameter nerve that transmits sensation from an entire region it is referred to as a nerve block. Nerve blocks are commonly used in dentistry, when the mandibular nerve is blocked for procedures on the lower teeth. With larger diameter nerves (such as the interscalene block for upper limbs or psoas compartment block for lower limbs) the nerve and position of the needle is localized with ultrasound or electrical stimulation. The use of ultrasound may reduce complication rates and improve quality, performance time, and time to onset of blocks. Because of the large amount of local anesthetic required to affect the nerve, the maximum dose of local anesethetic has to be considered. Nerve blocks are also used as a continuous infusion, following major surgery such as knee, hip and shoulder replacement surgery, and may be associated with lower complications. Nerve blocks are also associated with a lower risk of neurologic complications when compared to neuraxial blocks.:1639–1641
Central neuraxial anesthesia is the injection of local anesthetic around the spinal cord to provide analgesia in the abdomen, pelvis or lower extremities. It is divided into either spinal (injection into the subarachnoid space), epidural (injection outside of the subarachnoid space into the epidural space) and caudal (injection into the cauda equina or tail end of the spinal cord). Spinal and epidural are the most commonly used forms of central neuraxial blockade.
Spinal anesthesia is a "one-shot" injection that provides rapid onset and profound sensory anesthesia with lower doses of anesethetic, and is usually associated with neuromuscular blockade (loss of muscle control). Epidural anesthesia uses larger doses of anesthetic infused through an indwelling catheter which allows the anesthetic to be augmented should the effects begin to dissipate. Epidural anesethesia does not typically affect muscle control.
Because central neuraxial blockade causes arterial and vasodilation, a drop in blood pressure is common. This drop is largely dictated by the venous side of the circulatory system which holds 75% of the circulating blood volume. The physiologic effects are much greater when the block is placed above the 5th thoracic vertebra. An ineffective block is most often due to inadequate anxiolysis or sedation rather than a failure of the block itself.:1611
Pain that is well managed during and immediately after surgery improves the health of patients (by decreasing physiologic stress) and the potential for chronic pain. Nociception (pain sensation) is not hard-wired into the body. Instead, it is a dynamic process wherein persistent painful stimuli can sensitize the system and either make pain management difficult or promote the development of chronic pain. For this reason, preemptive acute pain management may reduce both acute and chronic pain and is tailored to the surgery, the environment in which it is given (in-patient/out-patient) and the individual patient.:2757
Pain management is classified into either pre-emptive or on-demand. On-demand pain medications typically include either opioid or non-steroidal anti-inflammatory drugs but can also make use of novel approaches such as inhaled nitrous oxide or ketamine. On demand drugs can be administered by a clinician ("as needed drug orders") or by the patient using patient-controlled analgesia (PCA). PCA has been shown to provide slightly better pain control and increased patient satisfaction when compared with conventional methods. Common preemptive approaches include epidural neuraxial blockade or nerve blocks. One review which looked at pain control after abdominal aortic surgery found that epidural blockade provides better pain relief (especially during movement) in the period up to three postoperative days. It reduces the duration of postoperative tracheal intubation by roughly half. The occurrence of prolonged postoperative mechanical ventilation and myocardial infarction is also reduced by epidural analgesia.
Risks and complications as they relate to anesthesia are classified as either morbidity (a disease or disorder that results from anesthesia) or mortality (death that results from anesthesia). Attempting to quantify how anesthesia contributes to morbidity and mortality can be difficult because a person's health prior to surgery and the complexity of the surgical procedure can also contribute to the risks.
Prior to anesthetic in the early 19th century, the physiologic stress from surgery caused significant complications and many deaths from shock. The faster the surgery was, the lower the rate of complications (leading to reports of very quick amputations). The advent of anesthesia allowed more complicated and life-saving surgery to be completed, decreased the physiologic stress of the surgery, but added an element of risk. It was two years after the introduction of ether anesthetics that the first death directly related to anesthetic was reported.
Morbidity can be major (myocardial infarction, pneumonia, pulmonary embolism, renal failure/insufficiency, postoperative cognitive dysfunction and allergy) or minor (minor nausea, vomiting, readmission). There is usually overlap in the contributing factors that lead to morbidity and mortality between the health of the patient, the surgery being performed and the anesthetic. To understand the relative risk of each contributing factor, consider that the rate of deaths totally attributed to the patient's health is 1:870. Compare that to the rate of deaths totally attributed to surgical factors (1:2860) or anesthesia alone (1:185,056) illustrating that the single greatest factor in anesthetic mortality is the health of the patient. These statistics can also be compared to the first such study on mortality in anesthesia from 1954, which reported a rate of death from all causes at 1:75 and a rate attributed to anesthesia alone at 1:2680.:993 Direct comparisons between mortality statistics cannot reliably be made over time and across countries because of differences in the stratification of risk factors, however, there is evidence that anesthetics have made a significant improvement in safety but to what degree is uncertain.
Rather than stating a flat rate of morbidity or mortality, many factors are reported as contributing to the relative risk of the procedure and anesthetic combined. For instance, an operation on a person who is between the ages of 60–79 years old places the patient at 2.32 times greater risk than someone less than 60 years old. Having an ASA score of 3, 4 or 5 places the person at 10.65 times greater risk than someone with an ASA score of 1 or 2. Other variables include age greater than 80 (3.29 times risk compared to those under 60), gender (females have a lower risk of 0.77), urgency of the procedure (emergencies have a 4.44 times greater risk), experience of the person completing the procedure (less than 8 years experience and/or less than 600 cases have a 1.06 times greater risk) and the type of anesthetic (regional anesthetics are lower risk than general anesthetics).:984 Obstetrical, the very young and the very old are all at greater risk of complication so extra precautions may need to be taken.:969–986
The immediate time after anesthesia is called emergence. Emergence from general anesthesia or sedation requires careful monitoring because there is still a risk of complication. Nausea and vomiting are reported at 9.8% but will vary with the type of anesthetic and procedure. There is a need for airway support in 6.8%, there can be urinary retention (more common in those over 50 years of age) and hypotension in 2.7%. Hypothermia, shivering and confusion are also common in the immediate post-operative period because of the lack of muscle movement (and subsequent lack of heat production) during the procedure.:2707
Postoperative cognitive dysfunction (also known as POCD and post-anesthetic confusion) is a disturbance in cognition after surgery. It may also be variably used to describe emergence delirium (immediate post-operative confusion) and early cognitive dysfunction (diminished cognitive function in the first post-operative week). Although the three entities (delirium, early POCD and long-term POCD) are separate, the presence of delirium post-operatively predicts the presence of early POCD. There does not appear to be an association between delirium or early POCD and long-term POCD. According to a recent study conducted at the David Geffen School of Medicine at UCLA, the brain navigates its way through a series of activity clusters, or "hubs" on its way back to consciousness. Dr. Andrew Hudson, an assistant professor in anesthesiology states, "Recovery from anesthesia is not simply the result of the anesthetic 'wearing off,' but also of the brain finding its way back through a maze of possible activity states to those that allow conscious experience. Put simply, the brain reboots itself."
Long-term postoperative cognitive dysfunction is a subtle deterioration in cognitive function, that can last for weeks, months, or longer. Most commonly, relatives of the person report a lack of attention, memory and loss of interest in activities previously dear to the person (such as crosswords). In a similar way, people in the workforce may report an inability to complete tasks at the same speed they could previously. There is good evidence that POCD occurs after cardiac surgery and the major reason for its occurrence is the formation of microemboli. POCD also appears to occur in non-cardiac surgery. Its causes in non-cardiac surgery are less clear but older age is a risk factor for its occurrence.:2805–2816
Throughout Europe, Asia, and the Americas a variety of Solanum species containing potent tropane alkaloids were used for anesthesia. In 13th century Italy, Theodoric Borgognoni used similar mixtures along with opiates to induce unconsciousness, and treatment with the combined alkaloids proved a mainstay of anesthesia until the nineteenth century. Local anesthetics were used in Inca civilization where shamans chewed coca leaves and performed operations on the skull while spitting into the wounds they had inflicted to anesthetize. Cocaine was later isolated and became the first effective local anesthetic. It was first used in 1859 by Karl Koller, at the suggestion of Sigmund Freud, in eye surgery in 1884. German surgeon August Bier (1861–1949) was the first to use cocaine for intrathecal anesthesia in 1898. Romanian surgeon Nicolae Racoviceanu-Piteşti (1860–1942) was the first to use opioids for intrathecal analgesia; he presented his experience in Paris in 1901.
Early Arab writings mention anesthesia by inhalation. This idea was the basis of the "soporific sponge" ("sleep sponge"), introduced by the Salerno school of medicine in the late twelfth century and by Ugo Borgognoni (1180–1258) in the thirteenth century. The sponge was promoted and described by Ugo's son and fellow surgeon, Theodoric Borgognoni (1205–1298). In this anesthetic method, a sponge was soaked in a dissolved solution of opium, mandragora, hemlock juice, and other substances. The sponge was then dried and stored; just before surgery the sponge was moistened and then held under the patient's nose. When all went well, the fumes rendered the patient unconscious.
The most famous anesthetic, ether, may have been synthesized as early as the 8th century, but it took many centuries for its anesthetic importance to be appreciated, even though the 16th century physician and polymath Paracelsus noted that chickens made to breathe it not only fell asleep but also felt no pain. By the early 19th century, ether was being used by humans, but only as a recreational drug.
Meanwhile, in 1772, English scientist Joseph Priestley discovered the gas nitrous oxide. Initially, people thought this gas to be lethal, even in small doses, like some other nitrogen oxides. However, in 1799, British chemist and inventor Humphry Davy decided to find out by experimenting on himself. To his astonishment he found that nitrous oxide made him laugh, so he nicknamed it laughing gas. Davy wrote about the potential anesthetic properties of nitrous oxide, but nobody at that time pursued the matter any further.
American physician Crawford W. Long noticed that his friends felt no pain when they injured themselves while staggering around under the influence of ether. He immediately thought of its potential in surgery. Conveniently, a participant in one of those “ether frolics", a student named James Venable, had two small tumors he wanted excised. But fearing the pain of surgery, Venable kept putting the operation off. Hence, Long suggested that he have his operation while under the influence of ether. Venable agreed, and on 30 March 1842 he underwent a painless operation. However, Long did not announce his discovery until 1849.
In 1846, Boston dentist William Thomas Green Morton conducted the first public demonstration of the inhalational anesthetic. Morton, who was unaware of Long's previous work, was invited to the Massachusetts General Hospital to demonstrate his new technique for painless surgery. After Morton had induced anesthesia, surgeon John Collins Warren removed a tumor from the neck of Edward Gilbert Abbott. This occurred in the surgical amphitheater now called the Ether Dome. The previously skeptical Warren was impressed and stated, "Gentlemen, this is no humbug." In a letter to Morton shortly thereafter, physician and writer Oliver Wendell Holmes, Sr. proposed naming the state produced "anesthesia", and the procedure an "anesthetic".
Morton at first attempted to hide the actual nature of his anesthetic substance, referring to it as Letheon. He received a US patent for his substance, but news of the successful anesthetic spread quickly by late 1846. Respected surgeons in Europe including Liston, Dieffenbach, Pirogov, and Syme quickly undertook numerous operations with ether. An American-born physician, Boott, encouraged London dentist James Robinson to perform a dental procedure on a Miss Lonsdale. This was the first case of an operator-anesthetist. On the same day, 19 December 1846, in Dumfries Royal Infirmary, Scotland, a Dr. Scott used ether for a surgical procedure. The first use of anesthesia in the Southern Hemisphere took place in Launceston, Tasmania, that same year. Drawbacks with ether such as excessive vomiting and its flammability led to its replacement in England with chloroform.
Discovered in 1831 by an American physician Samuel Guthrie (1782-1848), and independently a few months later by Frenchman Eugène Soubeiran (1797-1859) and Justus von Liebig (1803–73) in Germany, chloroform was named and chemically characterised in 1834 by Jean-Baptiste Dumas (1800–84). Its anaesthetic properties were noted early in 1847 by Marie-Jean-Pierre Flourens (1794-1867). The use of chloroform in anesthesia is linked to James Young Simpson, who, in a wide-ranging study of organic compounds, found chloroform's efficacy on 4 November 1847. Its use spread quickly and gained royal approval in 1853 when John Snow gave it to Queen Victoria during the birth of Prince Leopold. Unfortunately, though free of ether's flammability and consequent explosion hazard, chloroform is not as safe pharmacologically, especially when administered by an untrained practitioner (medical students, nurses, and occasionally members of the public were often pressed into giving anesthetics at this time). This led to many deaths from the use of chloroform that (with hindsight) might have been preventable. The first fatality directly attributed to chloroform anesthesia was recorded on 28 January 1848 after the death of Hannah Greener.
John Snow of London published articles from May 1848 onwards "On Narcotism by the Inhalation of Vapours" in the London Medical Gazette. Snow also involved himself in the production of equipment needed for the administration of inhalational anesthetics, the forerunner of today's anesthesia machines.
Of these first famous anesthetics, only nitrous oxide is still widely used today, with chloroform and ether having been replaced by safer but sometimes more expensive general anesthetics, and cocaine by more effective local anesthetics with less abuse potential.
Almost all healthcare providers use anesthesia to some degree, however most health professions have their own field of specialists in the field including medicine, nursing and dentistry.
Doctors specializing in perioperative care, development of an anesthetic plan, and the administration of anesthetics are known in the US as anesthesiologists and in the UK, Canada, Australia, and NZ as anaesthetists or anaesthesiologists. All anesthetics in the UK, Australia, New Zealand, Hong Kong and Japan are administered by doctors. Nurse anesthetists also administer anesthesia in 109 nations. In the US, 35% of anesthetics are provided by physicians in solo practice, about 55% are provided by anesthesia care teams (ACTs) with anesthesiologists medically directing anesthesiologist assistants or certified registered nurse anesthetists (CRNAs), and about 10% are provided by CRNAs in solo practice. There can also be anesthesiologist assistants (US) or physician assistant (anaesthesia) (UK) who assistant with anesthesia
There are many circumstances when anesthesia needs to be altered for special circumstances due to the procedure (such as in cardiac surgery, cardiothoracic anesthesiology or neurosurgery), the patient (such as in pediatric anesthesia, geriatric, bariatric or obstetrical anesthesia) or special circumstances (such as in trauma, prehospital care, robotic surgery or extreme environments).